1. Field of the Invention
The present invention relates to a shock absorber incorporated in a suspension system of a vehicle such as an automobile, and more particularly, to a damping force control system for such a shock absorber.
2. Description of the Prior Art
In Japanese Patent Laying-open Publication 61-287808 there is disclosed a damping force control system for a shock absorber incorporated in a suspension system of a vehicle such as an automobile which is adapted to switch over the damping force of the shock absorber to a high level when the relative displacement between the wheel and the vehicle body has increased beyond a predetermined limit value. In such a damping force control system the limit value for switching over the damping force of the shock absorber is determined from the view point of comfortableness in riding and stability in maneuvering while the durability of the shock absorber and the actuator therefor have not been much regarded. Therefore, when the vehicle runs on a rough road such as an unpaved road, the switching over of the damping force between a low level and a high level occurs at a very high frequency, causing a problem that the durability of the shock absorber and the actuator therefor is low.
In view of such a problem it has also been proposed in the above-mentioned Japanese patent laying-open publication to vary the limit value of the relative movement between the vehicle wheel and the vehicle body for switching over the damping force from a lower level to a higher level to be higher as the frequency of the switching over increases. By this arrangement the frequency of the switching over of the damping force between the lower level and the higher level decreases. However, as a result it occurs that the vehicle runs on a rough road for a long time with a lower damping force with a poor stability in maneuvering.